It is well known that helicopters are conventionally and mainly piloted, on the one hand, by controlling the general pitch of the blades of the main rotor, this control making it possible to vary the overall lift of the rotor, and, on the other hand, by controlling the same blades so as to make it possible to incline the rotor disc, the result of the combination of these two controls being the possibility of varying the amplitude and inclination of the mean resultant force generated by the rotor and consequently of piloting the helicopter which this rotor supports.
In most helicopters flying at the present time, these controls are introduced by means of a complex device with swashplates, these swashplates making it possible to generate simultaneously on the blades of the rotor, on the one hand, the control of the general pitch of the blades as a result of their axial shift along the axis of the rotor and, on the other hand, the sinusoidal control of the pitch or monocyclic control, the pulsation of which is equal to the rotational speed of the rotor, as a result of their angular tilting relative to the axis of the rotor.
In general, the swashplates used for controlling the variation of the pitch of the blades of a rotary-wing aircraft, such as a helicopter, are mounted round the rotor mast and comprise a rotary plate connected by means of links to the pitch levers of the rotor blades and driven in rotation about the rotor mast by the hub or the mast of the rotor and by means of at least one linkage, this rotary plate being mounted rotatably by means of at least one ball bearing on a non-rotary plate mounted round the rotor mast and sliding axially along the latter under the action of a collective-pitch lever actuated by the pilot, the non-rotary plate also being articulated relative to the rotor mast by means of a knuckle, in such a way that the plates can oscillate in all directions about the knuckle as a result of the action of connecting rods of the pilot controls on the non-rotary plate, this action being controlled from the cyclic control stick.
Moreover, the vibration levels recorded on helicopters are usually higher than those recorded on transport planes. The main cause of these high vibration levels is the main rotor of the helicopters, on which substantial alternating aerodynamic and dynamic forces are generated and, after being transmitted to the helicopter fuselage, result in high periodic accelerations, especially at high speed. Irrespective of the aim of achieving the best possible dynamic behavior of the rotors and structures, the means used hitherto for limiting these vibration phenomena are passive means of the anti-vibration or suspension type which are arranged in the region of the rotor head, at the connection between the rotor and fuselage and the fuselage itself. In parallel with the passive means, the future of which seems limited because their mass risks making it impossible to meet the increasingly stringent requirements of comfort associated with the increasingly high cruising speeds of modern helicopters, active means of controlling the vibrations are likewise being researched and developed.
As a particular instance of these active vibration control means, there has already been a proposal to use a multicyclic control, the principle of action of which, in this use, involves generating alternating forces which, in the region of the main rotor, oppose the vibration-generating forces.
In comparison with a monocyclic pitch control introduced by means of swashplates, this multicyclic control is a more elaborate control containing several harmonics of the rotational speed. It can also be used to seek an improvement of the flight qualities and performance gains, especially by controlling the stall on the lagging blade.
It should be noted, furthermore, that a multicyclic control of the main rotor of a helicopter, as a system for the active control of vibrations, is advantageously suitable for integration in a system of electrical flight controls, making it possible to obtain a generalized automatic control of the helicopter.
In a first known embodiment, it was proposed to introduce the multicyclic control in the region of the control jacks of the non-rotary plate of a conventional swashplate device, so that the multicyclic control is simply added to the conventional general and monocyclic control, corresponding to a fixed reference constant, that is to say without taking into account the rotation about the axis of the rotor. The only usefulness of such an embodiment is that it preserves the conventional monocyclic control chain with swashplates and pitch links, to which are added multicyclic linear actuators, such as linear hydraulic jacks, which are connected in series with the control jacks of the longitudinal shift and inclination of the non-rotary plate. Thus, by careful choice of the amplitude and phase of the commands sent to the three multicyclic control jacks, it is therefore possible to prevent the generation of vibrations in the region of the main rotor.
However, the disadvantage of such an embodiment is that linear actuators and their power control circuits have to be added to a swashplate device already heavy, bulky and complex itself, and that, furthermore, it is necessary to reinforce the swashplates themselves to a considerable extent. This therefore means an increase in cost, complexity and weight of such an installation mounted round the rotor mast.
In contrast to such a superposition of a conventional monocyclic control and a multicyclic control, there has already been a proposal to obtain the multicyclic control by means of a direct rotating-reference control via actuating devices which each rotate together with the blade of which the pitch is controlled.
Various embodiments of this type were presented in an article by Mr. Kenneth F. Guinn, entitled "Individual blade control independent of a swashplate" and published in the July 1982 issue of the "Journal of American Helicopter Society".
This article presents embodiments of the control of a main helicopter rotor which depart from the conventional concept by placing control actuators, corresponding power supplies and computers in the rotary control system. In a first embodiment, the purpose of which is to improve the mechanical pitch-control device and which functions as a means making it possible to introduce a higher-harmonic control into the rotor, the multicyclic control comprises an actuator, namely a linear hydraulic jack, mounted axially along the rotor mast and outside the upper part of the latter, the hydraulic supply circuit and the pump being accommodated inside the rotor mast. Whereas, in this first embodiment, the multicyclic control is used in parallel with the monocyclic control, the other embodiments proposed in this article no longer have swashplates allowing the use of a monocyclic control, and some of these embodiments possess series arrangements of a linear hydraulic jack, its hydraulic supply means, especially the pump, and a connecting rod for actuating the pitch-control lever of the corresponding blade, these elongate arrangements being placed axially either on the inside of the rotor mast or on the outside and along the latter, depending on whether the jacks are to be protected against the ambient medium, irrespective of the thermal problems presented by the functioning of these devices, or whether it is desirable to profit from the rotary movement of the jacks to obtain a ventilator effect which improves their cooling. In another configuration, to ensure that the cooling of the controls during high-speed operation when the temperature of the system is at a maximum is sufficient because of the large volume of displaced air, the controls are placed on the rotor hub and comprise, for each blade, a hydraulic pump mounted on the central part of the hub, fed via a conduit within the rotor mast and itself feeding a linear hydraulic jack which is fastened spanwise to a radial arm of the hub and of which the outer radial end in relation to the axis of rotation of the rotor actuates two bent pitch-control levers for controlling the pivotings of the blade about its pitch-changing axis. In this configuration, the double lever mechanisms actuating the blade require supports which are extremely long and therefore heavy and not very rigid, the jacks are fed via relatively long pipelines and the centrifugal forces subject the structures to stress and influence the functioning of the elements of the device mounted in the direction of the span. In another configuration, the actuators are still linear jacks, but these are arranged in the direction of the chord of the blade, thus making it possible to mount them together with their lever mechanisms nearer to the rotor mast and reduce the weight of the hydraulic pipelines and supports of the lever mechanisms. An alternative version of this configuration which still uses jacks mounted in the direction of the chord is described in U.S. Pat. No. 4,379,678 and eliminates all the bent levers and the lever supports by means of which the jacks actuate the levers for controlling the pitch of the blades, but at the expense of a mounting of the jacks on bulky transverse articulated supports. In the abovementioned U.S. Patent, the rotor equipped with a device for the individual control of the pitch of each blade is a rotor on which each blade has its root coupled to the outer radial end of a flexurally and torsionally pliant arm of the hub, driven in rotation by a tubular rotor mast. An electrohydraulic actuator which is a triple linear jack is arranged along the chord, perpendicularly to the axis of the blade, in the region of the root of the latter, and the rod of the jack acts as a pitch-controlling link actuating a pitch-control lever fixed to the blade root. Each of the three stages of the electrohydraulic jack is connected to an independent hydraulic circuit comprising a pump and a hydraulic tank, and each of the three stages of the jack is controlled simultaneously, independently of the other two stages, by an electrical-signal generator which sends control commands to the servo valves associated with the jacks, the signal generator being supplied with electrical current produced by an alternator which is driven in rotation, at the same time as a hydraulic pump, by means of a common shaft carrying a drive pinion in engagement with a gear wheel fastened to the upper end of a fixed (non-rotary) standpipe passing through the tubular rotor mast. Since the electrohydraulic jack, each electrical-signal generator, each corresponding alternator and each hydraulic pump of the three independent circuits, each feeding one of the stages of the jack, are fastened to the rotor hub and therefore driven in rotation together with the latter, each pump and each corresponding alternator are driven as a result of the rotation of the common drive pinion about the fixed gear wheel. The transverse shifts of the rod of the jack oriented along the chord are transmitted to a bent pitch lever by means of a U-shaped transmission linkage, the base of which is connected to the rod of the jack and the two wings of which extend on either side of the jack and have their free end coupled to one arm of the bent pitch lever. The other arm of this lever is integral in terms of rotation with the inner radial end of a radial extension of the blade which prolongs the latter, towards the rotor hub, beyond a lag hinge which articulates the blade on the two radial branches of the corresponding hub arm shaped as a yoke with two flexurally and torsionally pliant branches, between which the blade extension is accommodated. Furthermore, a linear shock absorber for damping the angular oscillations of the lagging blade is coupled between two offset fastenings, of which one is on the blade root and the other on the blade extension.
Because of the crowding caused on the rotor head by the presence not only of the jacks mounted along the chord, but also of the corresponding hydraulic circuits, together with the associated pipelines, pumps, tanks, filters and valves, and of the electrical and optical circuits for generating and transmitting control commands, it proved necessary to provide a fairing for the rotor hub, in order to protect the controls and reduce the aerodynamic drag arising as a result of their presence on the hub. Furthermore, the hydraulic pipelines were accommodated at least partially in this hub fairing, so that the latter at the same time functions as a heat exchanger.
However, such a system of complex structure presents considerable problems of mounting, alignment and also wear of the gaskets in the region of the linear jacks used. For these reasons, the abovementioned review article proposes another configuration having a more efficient jack arrangement. This configuration comprises two pitch-control levers and four linear jacks for each blade. Each blade jack is controlled by a different hydraulic and electrical power package which is located on the rotor hub. Four computers, likewise placed on the hub, process all the control data going to the rotor blades or coming from these and originating from blade-position detectors. The hydraulic power packages are driven by fixed pinions mounted on standpipes. One standpipe is placed in the tubular rotor mast and comes out above the rotor hub, and another standpipe is placed outside the mast and comes just under the hub, so that when the rotor rotates the hydraulic power packages are driven by gear trains, themselves driven in rotation about fixed pinions. Two of the four hydraulic power packages are provided above the hub and the other two below it. The control commands from the electrohydraulic servo valves equipping the four linear jacks, and the blade configuration or blade position data are transmitted via an interface between the rotary part of the rotor and the non-rotary part, by means of optical fibers interacting with redundant optical collectors, of which one is located above the rotor hub and the other below it. As regards the driving of the blade in rotation about its pitch-changing axis, each blade is integral in terms of rotation with two pitch-control levers, perpendicular to the pitch axis of the blade and to its drag plane, of which one extends above this drag plane and the other below it. The free end of each of the two pitch-control levers is articulated on the rods of two linear jacks mounted along the chord, in opposition, one fore of and the other aft of the pitch axis of the blade. The four jacks of each blade are thus distributed between two upper jacks and two lower jacks which are all arranged along the chord of the blade and in paired opposition, in order to control the pitch-control tilts about the pitch-changing axis of the blade and thus bring about the angular variations of the blade about this axis. Each of the four linear jacks associated with each blade possesses an electrohydraulic control servo valve, shutter valves and a branch circuit for limiting the loads experienced, and also linear transducers for the position of the piston of the jack and of the overload shutter valves. All the jacks are accommodated in two jack supports which also perform the function of hydraulic collectors, with quick-acting connections for connecting the jacks to the corresponding hydraulic circuits at the time of assembly. Thus, four rotary electrohydraulic servo controls are used for each blade, their other remaining components, namely a tank, a filter module and one or two computers, being combined to form an integral electrohydraulic unit which is placed near the associated pump and alternator. A hub fairing comprising two matching convex discs is fastened to the rotor hub, in order to fair the latter and protect the controls.
In comparison with a conventional swashplate device which, arranged round the rotor mast between the rotor and the fuselage, causes considerable drag in cruising flight, placing the multicyclic flight controls on the rotor hub with a suitable fairing and eliminating the swashplates and the corresponding control linkages make it possible to reduce the rotor drag to an appreciable extent. Furthermore, above all, the individual blade control ensures high control flexibility by making it possible to introduce complex control laws at the same time as the conventional general and cyclic pitch inputs to the blades. This makes it easier to use both harmonic and non-harmonic control inputs in order to improve the operating loads of the rotor, the vibration qualities, the performances and/or the maneuverability capacities of the craft. It is possible in this way to damp the excitations of the blades before they can cause undesirable vibrations, and by means of higher harmonics it is possible to bring about elimination or breakaway of ice deposited on the blades and thus ensure deicing.
However, although single-acting jacks of the non-compensated type can be used in this configuration, it is nonetheless true that these advantageous effects arise as a result of the use of a complex structure comprising, for each blade, four rotary electrohydraulic servo controls installed above and below the rotor hub and the blade roots and also round the upper part of the rotor mast, and therefore such an installation remains highly complex, costly, bulky and of untried reliability.
The same applies essentially to the system for the individual control of the pitch of the blades of a helicopter rotor which is described in U.S. Pat. No. 4,519,743. In this patent, the root of each blade, the pitch of which is to be controlled individually, is retained in a metal yoke fixed to the inner radial reinforcement of a laminated ball joint, the outer radial reinforcement of which is retained on a rigid supporting arm fixed to the hub. In the region of the laminated ball joint which allows the movements of the blade in terms of flapping and drag and about its pitch-changing axis, a flexible metal strip connecting the supporting arm of the hub to the retaining yoke of the blade root carries strain gages which supply output signals indicating the torsional deformation of the blade about its pitch axis and under flapping. The dynamic response of the blade can thus be monitored in order to prepare negative feedback signals in the control loop which, moreover, includes essentially electromechanical means for adjusting the angular position of the blade about its pitch-changing axis. Furthermore, at least one accelerometer is mounted on the blade, in such a way that it is sensitive to the accelerations in a direction perpendicular to the surface of the blade, so as to be sensitive to the deformations under flapping and supply negative feedback signals likewise fed into the control loop. The pitch of each blade of the rotor is controlled individually by a servo-motor belonging to each blade and by means of a mechanical transmission comprising gears and a pinion/crank assembly. The servo-motor is a motor/tachometer assembly mounted parallel to the axis of the rotor mast between two radial flanges supported by the latter. A pinion mounted on the output shaft of the motor is in engagement with a bevel wheel which drives in rotation a radial shaft mounted pivotably in bearings carried by supports fixed to the rotor hub, and this radial shaft drives in rotation a crank, the end of which carries a crank pin articulated on the lower end of a pitch-control link, the upper end of which is articulated on a transverse diametral axle intersecting the pitch-changing axis of the blade and fastened to the retaining yoke of the root of this blade and to the inner radial reinforcement of the laminated ball joint. Thus, any rotation of the motor in one direction or the other is transmitted, via output pinions of the motor, to the crank of which the crank pin exerts a pull or a push on the pitch-control link, in order thereby to control the angular variations of the blade about its pitch-changing axis.
The disadvantages of such a solution are that the position of the motor assemblies along the rotor mast and on the outside of this and the presence of the reduction-pinion stages, the supports of their bearings and the crank/crank pin assemblies and pitch-control links on a plate of the rotor hub give the device a considerable bulk which causes a likewise considerable aerodynamic drag and which adds to the complexity of the structure, having adverse effects on the production and maintenance costs.